I made it through this fight by letting Worm and bishop fight each other while moving my party (well half of the party, 2 died) away from their agro range. Until Worm appeared bishop was stunlocked (frozen, stunned, teleported out), while i was killing secondary enemies (2 of my party members died during the process, but dont remember exactly how).
By the way fight between Worm and Alexandar was kind of funny, because Disease effect is bugged. When it apllies, it heals you to max health. And when it wears off it heals you to max health too.
So i had to wait for the right moment when they were both low on hp to jump in there with my fighter on Rage and finish them off
PS: actually Alexandar won the duel and i finished him off after that
